1. Field of the Invention
This invention relates to apparatus used in the recovery of silver from solutions such as spent photographic processing solutions. In particular, the invention relates to a cell configuration in silver recovery apparatus of the type comprising an anode-cathode cell for electrolytic recovery of silver, and more particularly to such apparatus of the type having a rotating cathode.
Recovery of silver is of course highly desireable from an economic viewpoint. Environmental factors are also extremely important, however. A high recovery efficiency is needed in order to meet or exceed the effluent standards established by agencies such as the Environmental Protection Agency in the United States.
Spent photographic processing solutions contain large quantities of silver. In such solutions, the dissolved silver is typically in a complex with either sodium or ammonium thiosulfate. There are also other sulfur compounds present, e.g. sodium sulfite and sodium bisulfite.
One particular commonly-used solution is a so-called "bleach-fix" solution, comprising a fixing agent which dissolves silver halides from the photographic film being processed, and a bleaching agent, which converts metallic silver into a silver halide. By the time the solution is spent and exits the photographic processing equipment, the bleaching agent is generally inactive, but it can be easily regenerated by oxidation.
Many forms of equipment exist for recovering this valuable silver from the solutions. It is well known that silver can be recovered from spent photographic solutions by using an electrolytic cell, i.e. by passing a direct current between electrodes immersed in the solution. The ionic silver, having a low reduction potential, is readily reduced to metallic silver at the cathode. The direct current applied to the electrodes causes the silver in the spent solution to deposit onto the cathode.
For maximum efficiency and speed in recovering the silver from bleach-fix solutions in particular, a high current density should be achieved at the cathode. However, reactions take place at the anode as well, and these reactions must be taken into account. These reactions can significantly affect the silver recovery process if the applied voltage becomes too high, in which case gases are evolved at both electrodes, and elemental sulfur is produced at the anode. This sulfur causes a problem with the silver recovery because it readily combines with the silver to form silver sulfide, which is insoluable. The silver sulfide forms a colloidial suspension, making it very difficult to reclaim the silver. This problem is commonly referred to as "sulfiding".
Another important factor in the overall efficiency of a recovery cell, as well as in reducing the tendency for sulfiding, is the conductance of the cell. The solution has a specific conductivity which does not change substantially until low silver concentrations are reached, unless salts are added. It is desireable to have a high conductance, so that a large current can be passed through the solution without applying a large voltage. The high current is desireable for achieving good recovery rates, and the low voltage is desireable to minimize other reactions.
As a natural result of recovering the silver, and to a lesser extent as a result of the other reactions, the conductivity of the solution decreases as the silver concentration reaches low levels. This causes the voltage to rise and current to drop, when using an unregulated power supply, resulting in decreased efficiency and increased sulfiding. It is not desireable to use a voltage regulated supply, however.
If stationary plate electrodes are used without agitation, then increased current density or decreased silver concentration results in very unsatisfactory current efficiency due to the depletion of silver within the boundary layer at the cathode surface, i.e. due to poor mass transfer. Furthermore, the low concentration of silver causes the voltage to rise thus resulting in excessive sulfiding. Current efficiency is defined such that at 100% current efficiency, one equivalent of silver, 108 grams, is deposited on the cathode for each faraday of current passed through the cell. This equates to 4.025 grams of silver per ampere-hour, or 7.72 ampere-hours per Troy ounce.
The probability of sulfiding when the silver concentration becomes low is further increased because of the problem of getting silver ions to the cathode surface, which is essentially a mass transfer problem. Mass transfer is an important factor in achieving high space-time yields. By mass transfer is meant the rate at which silver-laden solution can be transferred to the cathode. Space-time yield is a measure of the amount of silver recovered for a given apparatus or tank size over a given time.
It has been hypothesized that the rate of silver recovery is limited by the applied current at high concentrations of silver, and by the rate of diffusion of silver ions to the cathode at low silver concentrations. The rate is thus said to be current limited at high silver concentrations and transport limited at low silver concentrations. For any given apparatus and batch type, there is a well-defined silver concentration at which the transition from current-limited to transport-limited silver recovery rate occurs.
If stationary plates are used for electrodes in a tank of fixer, very low current densities must be used to prevent excessive sulfiding. This naturally results in very low current efficiency. To maintain efficient recovery of silver with nothing more than diffusion-limited mass transfer, i.e. mass transfer which is dependent on diffusion rather than forced circulation, a very large electrode surface area would be required to maintain low current densities.
By improving the mass transfer characteristics of the cell, recovery efficiency can be improved, because the silver can be reclaimed down to very low concentrations without excessive sulfiding. More efficient use of space and time can also be achieved if reduction in the size of the recovery cell can be achieved and if higher currents can be utilized.
The challenge for the equipment designer is essentially to achieve and sustain a high current efficiency and high space-time yield without excessive sulfiding.
2. Description of the Prior Art
In the prior art, few serious attempts have been made to develop new approaches to dealing with these problems.
One of the most widely utilized cell configurations is the "rotating cathode" design, the cathode usually consisting of stainless steel discs or a stainless steel drum rotating in the centre of a tank, a current source being connected to this cathode. Graphite plates carried on the inside perimeter of the tank directly adjacent to the cathode surfaces act as the anodes. This rotating cathode configuration allows the current density at the cathode surface to be increased dramatically compared to the current density in cells having stationary electrodes. This is primarily because of the smaller boundary layer at the cathode surface, which results in improved mass transfer. There is also an improvement in mass transfer because of the mixing of solution produced by the drag of the cathode rotating in the solution.
With minor changes in materials and dimensions, the simple rotating cathode configuration has remained the state of the art in silver recovery technology to date. Significant improvements generally have not been pursued vigorously, presumably because with a conventional rotating cathode and regular fixing solutions, current efficiencies on the order of 90 percent have been achieved. However, conventional rotating cathode configurations cannot achieve such high current efficiency in the case of the so-called "bleach-fix" solutions.